Preventing cavities could one day involve the dental equivalent
of a military surgical strike. A team of researchers supported
by the National Institute of Dental and Craniofacial Research report
they have created a new smart anti-microbial treatment that can
be chemically programmed in the laboratory to seek out and kill
a specific cavity-causing species of bacteria, leaving the good
bacteria untouched.

The experimental treatment, reported online in the journal Antimicrobial
Agents and Chemotherapy, is called a STAMP. The acronym stands
for “specifically targeted antimicrobial peptides” and, like its
postal namesake, STAMPs have a two-sided structure. The first is
the short homing sequence of a pheromone, a signaling chemical
that can be as unique as a fingerprint to a bacterium and assures
the STAMP will find its target. The second is a small anti-microbial
bomb that is chemically linked to the homing sequence and kills
the bacterium upon delivery.

While scientists have succeeded in the past in targeting specific
bacteria in the laboratory, this report is unique because of the
STAMPs themselves. They generally consist of less than 25 amino
acids, a relative pipsqueak compared to the bulky, bacteria-seeking
antibodies that have fascinated scientists for years. Because of
their streamlined design, STAMPs also can be efficiently and rapidly
produced on automated solid-phase chemistry machines designed to
synthesize small molecules under 100 amino acids, called peptides.

The first-generation STAMPs also proved extremely effective in
the initial laboratory work. As reported in this month’s paper,
the scientists found they could eliminate the cavity-associated
oral bacterium Steptococcus mutans within 30 seconds from an oral
biofilm without any collateral damage to related but non pathogenic
species attached nearby. Biofilms are complex, multi-layered microbial
communities that routinely form on our teeth and organs throughout
the body. According to one estimate, biofilms may be involved to
varying degrees in up to 80 percent of human infections.

“We’ve already moved the S. mutans STAMP into human studies, where
it can be applied as part of a paste or mouthrinse,” said Dr. Wenyuan
Shi, senior author on the paper and a scientist at the University
of California at Los Angeles School of Dentistry. “We’re also developing
other dental STAMPs that target the specific oral microbes involved
in periodontal disease and possibly even halitosis. Thereafter,
we hope to pursue possible medical applications of this technology.”

Shi said his group’s work on a targeted dental therapy began about
eight years ago with the recognition that everyday dental care
had reached a crossroads. “The standard way to combat bacterial
infections is through vaccination, antibiotics, and/or hygienic
care,” said Shi. “They represent three of the greatest public-health
discoveries of the 20th century, but each has its limitations in
the mouth. Take vaccination. We can generate antibodies in the
blood against S. mutans. But in the mouth, where S.
mutans lives
and our innate immunity is much weaker, generating a strong immune
response has been challenging.”

According to Shi, a major limitation of antibiotics and standard
dental hygiene is their lack of selectivity. “At least 700 bacterial
species are now known to inhabit the mouth,” said Shi. “The good
bacteria are mixed in with the bad ones, and our current treatments
simply clear everything away. That can be a problem because we
have data to show that the pathogens grow back first. They’re extremely
competitive, and that’s what makes them pathogenic.”

To illustrate this point, Shi offered an analogy. “Think of a
lawn infested with dandelions,” he said. “If you use a general
herbicide and kill everything there, the dandelions will come back
first. But if you use a dandelion-specific killer and let the grass
fill in the lawn, the dandelions won’t come back.”

Hoping to solve the selectivity issue, Shi and his colleagues
began attaching toxins to the homing region of antibodies. They
borrowed the concept from immunotherapy, an area of cancer research
in which toxin-toting antibodies are programmed to kill tumor cells
and leave the nearby normal cells alone.

Despite some success in killing specific bacteria in the oral
biofilm, Shi said his group soon encountered the same technical
difficulty that cancer researchers initially ran into with immunotherapy.
Their targeting antibodies were large and bulky, making them unstable,
therapeutically inefficient, and expensive to produce. “That’s
when we decided to get higher tech,” said Dr. Randal Eckert, a
UCLA scientist and lead author on the study.

Or, as Eckert noted, that’s when they turned to the power of genomics,
or the comparative study of DNA among species. Eckert and colleagues
clicked onto an online database that contains the complete DNA
sequence of S. mutans. They identified a 21-peptide pheromone called “competence
stimulating peptide,” or CSP, that was specific to the bacterium.
From there, they typed instructions into an automated solid-phase
chemistry machine to synthesize at once the full-length CSP and
a 16-peptide anti-microbial sequence, and out came their first
batch of STAMPs.

After some trial and error, Eckert said he and his colleagues
decided “to get even shorter.” They ultimately generated a STAMP
with the same anti-microbial agent but with a signature eight-peptide
CSP sequence to target S. mutans. “We pooled saliva from five people
and created an oral biofilm in the laboratory that included a couple
hundred species of bacteria,” said Eckert. “We applied the STAMP,
and it took only about 30 seconds to eliminate the S. mutans in
the mixture, while leaving the other bacteria in tact.”

As dentists sometimes wonder, what would happen if S. mutans is
eliminated from the oral biofilm? Does another equally or more
destructive species fill its void, creating a new set of oral problems?
Shi said nature already provides a good answer. “About 10 to 15
percent of people don’t have S. mutans in their biofilms, and they
do just fine without it,” he said. “Besides, S. mutans is not a
dominant species in the biofilm. It only becomes a problem when
we eat a lot of carbohydrates.”

Looking to the future, Shi said new STAMPs that seek out other
potentially harmful bacterial species could be generated in a matter
of days. He said all that is needed is the full DNA sequence of
a microbe, a unique homing sequence from a pheromone, and an appropriate
anti-microbial peptide. “We have a collection of anti-microbial
peptides that we usually screen the bacterium through first in
the laboratory,” said Shi. “We can employ the anti-microbial equivalent
of either a 2,000-ton bomb or a 200-pound bomb. Our choice is usually
somewhere in the middle. If the anti-microbial peptide is too strong,
it will also kill the surrounding bacteria, so we have to be very
careful.”

This research also was supported by a University of California
Discovery Grant, Delta Dental of Washington, Delta Dental of Wisconsin,
and C3 Jian Corporation. The National Institute of Dental and Craniofacial
Research is the nation’s leading funder of research on oral, dental,
and craniofacial health. For more information, visit the Web site
at http://www.nidcr.nih.gov/.

The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit www.nih.gov.